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I've learned that in almost all cases, stock is best for FE. The header I have helps torque in too high of an rpm range, however it is lighter and more durable than the original cast iron manifold which cracks easily. It's a compromise I was willing to take for the one or two times a month I go WOT. A light weight header option could be a stock Focus Zetec header, it's small and light and you can affix a nice small light cat to it (at some point this is the route I will most likely take). The stock pipe size will not hurt FE, in fact it will be lighter than any 2.25 or 2.5" cat back exhaust you put on. (I will probably go to a lighter exhaust in the future as well)
You can pretty much leave airflow through the engine alone and focus on parasitic losses, weight reduction, rolling resistance and aero resistance. There is a lot of low hanging fruit here that is much easier and cheaper to get than a full performance exhaust.
I have a wideband and with no MAF fooling happing, it idles at a perfect 14.7:1, I'm not sure where you got 13.5 from, not even in open loop in cold weather does it idle below 14.7, doesn't seem like it has a cold start up enrichment. When I leaned it out to just above 15:1 I started to have problems with idle fluctuation and surging, it was really annoying, after many trials I found it just didn't give the results I was hoping for in fuel savings. At one point I got the idle gph down to 0.08, then I did the math on the actual distance I was coasting (about 3.4 miles each way) and realized the savings was so minimal it was not worth all the hassle trying to get it work right. When I did the math on DFCO or EOC, I found a much better pay off for the work. Doing the homework there really helped me refine my driving and my commute route
If your serious about lean burn, you need to start with a wideband and I'd suggest jumping right into some tuning software, don't go the route I did, it's quite a headache. You'll pay back the software in a year.
Watch your Keman Intake (PVC Pipe), it will warp from heat in about a year an start throwing lean codes.
Focus on parasitic losses, weight reduction, rolling resistance and aero resistance. They all work together and start to add up quickly, get after the aero and you will find your opportunities to DFCO will improve and lengthen, you could even wire up an engine kill switch so you can open the throttle 100% and stop the pumping losses that are slowing you down so much. Alternator kill switches are good for helping reduce parasitic losses as well as underdrive pulley's and P/S & A/C deletes. You gotta get some RE92's to really understand what they can do for your FE, they are almost magic. Sell what you have and get some as soon as possible. Super light wheels help also. Lean burn is a more advanced technique and I'd recommend you build a good foundation with a focus on parasitic losses, weight reduction, rolling resistance and aero resistance and when you go lean it will work a loooot better.
:-)
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